Question: For his birthday, Bert gets a box that holds $125$ jellybeans when filled to capacity. A few weeks later, Carrie gets a larger box full of jellybeans. Her box is twice as high, twice as wide and twice as long as Bert's. Approximately, how many jellybeans did Carrie get?
Explanation: Since the exact dimensions of Bert's box do not matter, assume the box is $1 \times 2 \times 3$.  Its volume is 6.  Carrie's box is $2 \times 4 \times 6$, so its volume is 48 or 8 times the volume of Bert's box.  Carrie has approximately $8(125) = \boxed{1000}$ jellybeans.
$\textbf{Note:}$ Other examples may help to see that the ratio is always 8 to 1.